The present invention generally relates to signal pickup devices in rotary recording medium reproducing apparatuses, and more particularly to a signal pickup device capable of controlling the tracking accurately and efficiently. The tracking control is performed in a manner such that a cantilever rotates to displace a reproducing element which reproduces recorded signals from a rotary recording medium in a direction perpendicular to the longitudinal direction of tracks of the rotary recording medium to follow and trace the same.
Heretofore, there have been apparatus of the designed type. For example, a rotary disc (referred to as "disc" hereinafter) has a video signal recorded on a spiral track as variations in the geometrical shapes corresponding to an information content. A reproducing element is caused to trace over the spiral track and reproduce the recorded video signal. In a pickup device of this character, it is necessary for the signal pickup device to trace the track accurately. For this reason, it is necessary to provide means for detecting any tracking deviation of the signal pickup device relative to the track. In response to this error, the position of the signal pickup device is controlled so that it will trace accurately over the track thereby accomplishing a tracking control.
The present applicant has previously described in a commonly assigned U.S. patent application. Ser. No. 885,579, filed Mar. 13, 1978 (U.S. Pat. No. 4,170,783) by Osamu Tajima, and entitled "Signal pickup device for reproducing an information signal recorded on a track of a rotary recording medium" a signal pickup device in which a permanent magnet member of a rectangular parallelepiped shape and having magnetic poles on the opposite lateral faces thereof is fixed to the proximal end of a cantilever, the cantilever is supported by an elastic support member at a separated point in front of the permanent magnet so as to be rotatable, and a tracking coil is disposed in the rear position with respect to the elastic support member so as to surround the permanent magnet member. On tracking control operation, a torque is produced to act on a cantilever assembly comprising the cantilever, the reproducing element, and the permanent magnet member.
A positional relationship between the permanent magnet member, the elastic support member, and the tracking coil causes a center of the torque to be located at a position which is far apart backwards (in a direction opposite to the reproducing element) with respect to the point where the elastic support member supports the cantilever.
In this connection, the torque for tracking control acts on the elastic support member is an extending (or axial) direction thereof, and the tracking control inevitably accompanies compression and stretching deformation in the axial direction of the elastic support member.
This signal pickup device is accompanied by various problems such that 1 the elastic support member is brought into resonance at a specific frequency, whereby a tracking servo system is subjected to phase delay, thus deteriorating tracking accuracy, and 2 a large torque sufficient to compress and stretch the elastic support member is required, which results in a substantial decrease in efficiency of the tracking control operation.